Electronic devices may be fabricated using a sequence of processes to form successive device layers on a substrate such as a silicon wafer. The successive device layers may be patterned using lithography, where the term lithography refers to techniques in which light (or particles such as electrons or ions) is used to transfer a pattern from a mask or reticle to a substrate.
Reducing the feature sizes of semiconductor devices allows for more functionality to be provided in a smaller area. However, smaller feature sizes pose significant lithography challenges. For example, as feature sizes decrease, the resolution of the lithography system must be sufficient to resolve them.
The resolution of a lithography system depends on a number of factors. One factor is the angle of the light used to image the features. Larger angles allow for higher resolution (and thus enable smaller device features).
Some existing systems and techniques may be used to improve resolution. These include off-axis illumination (OAI) and the use of phase shifting masks, such as embedded phase shift masks (EPSMs).
Like reference symbols in the various drawings indicate like elements.